Pure Java, nothing else

Monthly Archives: March 2019

Introduction

POM files are XML formatted files that declaratively describe the build structure of a Java project to be built using Maven. Maintaining the POM XML files of large Java projects is many times cumbersome. XML is verbose and also the structure of the POM requires the maintenance of redundant information. The naming of the artifacts many times is redundant repeating some part of the name in the groupId and in the artifactId. The version of the project should appear in many files in case of a multi-module project. Some of the repetitions can be reduced using properties defined in the parent pom, but you still have to define the parent pom version in each module pom, because you refer to a POM by the artifact coordinates and not just referring to it as “the pom that is there in the parent directory”. The parameters of the dependencies and the plugins can be configured in the parent POM in the pluginManagement and dependency management but you still can not get rid of the list of the plugins and dependencies in each and every module POM though they are usually just the same.

You may argue with me because it is also the matter of taste, but for me, POM files in their XML format are just too redundant and hard to read. Maybe I am not meticulous enough but many times I miss some errors in my POM files and have a hard time to fix them.

There are some technologies to support other formats, but they are not widely used. One such approach to get rid of the XML is Poyglot Maven. However, if you look on that Github page at the very first example, which is Ruby format POM you can still see a lot of redundant information, repetitions. This is because Polyglot Maven plugs-into Maven itself and replaces only the XML format to something different but does not help on the redundancy of the POM structure itself.

In this article, I will describe an approach that I found much better than any other solution, where the POM files remain XML for the build process, thus there is no need for any new plugin or change of the build process, but these pom.xml files are generated using the Jamal macro language from the pom.xml.jam file and some extra macro files that are shared by the modules.

Jamal

The idea is to use a text-based macro language to generate the XML files from some source file that contains the same information is a reduced format. This is some kind of programming. The macro description is a program that outputs the verbose XML format. When the macro language is powerful enough the source code can be descriptive enough and not too verbose. My choice was Jamal. To be honest, one of the reasons to select Jamal was that it is a macro language that I developed almost 20 years ago using Perl and a half year ago I reimplemented it in Java.

The language itself is very simple. Text and macros are mixed together and the output is the text and the result of the macros. The macros start with the { character or any other string that is configured and end by the corresponding } character or by the string that was configured to be the ending string. Macros can be nested and there is fine control what order the nested macros should be evaluated. There are user-defined and built-in macros. One of the built-in macros is define that is used to define user-defined macros.

An example talks better. Let’s have a look at the following test.txt.jam file.

I deleted the empty lines manually for typesetting reasons though, but you get a general idea. GAV is defined using the built-in macro define. It has three arguments named _groupId,_artifactId and _version. When the macro is used the format argument names in the body of the macro are replaced with the actual values and replace the user-defined macro in the text. The text of the define built-in macro itself is an empty string. There is a special meaning when to use @ and when to use # in front of the built-in macros, but in this article, I cannot get into that level of detail.

The if macros also make it possible to omit groupId, artifactId or version, thus

Be warned that this needs an insane level of understanding of macro evaluation order, but as an example, it shows the power. More information on Jamal https://github.com/verhas/jamal

Let’s get back to the original topic: how Jamal can be used to maintain POM files.

Cooking pom to jam

There can be many ways, which each may be just good. Here I describe the first approach I used for the Java::Geci project. I create a pom.jim file (jim stands for Jamal imported or included files). This contains the definitions of macros, like GAV, dependencies, dependency and many others. You can download this file from the Java::Geci source code repo: https://github.com/verhas/javageci The pom.jim file can be the same for all projects, there is no any project specific in it. There is also a version.jim file that contains the macro that defines at one single place the project version, the version of Java I use in the project and the groupId for the project. When I bump the release number from -SNAPSHOT to the next release or from the release to the next -SNAPSHOT this is the only place where I need to change it and the macro can be used to refer to the project version in the top level POM? but also in the module POMs referring to the parent.

In every directory, where there should a pom.xml file I create a pom.xml.jam file. This file imports the pom.jim file, so the macros defined there can be used in it. As an example the Java::Geci javageci-engine module pom.xml.jam file is the following:

Having this I can start Maven with the command line mvn -f genpom.xml clear. This not only creates all the POM files but also clears the previous compilation result of the project, which is probably a good idea when the POM file changes. It can also be executed when there is no pom.xml yet in the directory or when the file is not valid due to some bug you may have in the jam cooked POM file. Unfortunately, all recursivity has to end somewhere and it is not feasible, though possible to maintain the genpom.xml as a jam cooked POM file.

Summary

What I described is one approach to use a macro language as a source instead of raw editing the pom.xml file. The advantage is the shorter and simpler project definition. The disadvantage is the extra POM generation step, which is manual and not part of the build process. You also lose the possibility to use the Maven release plugin directly since that plugin modifies the POM file. I myself always had problems to use that plugin, but it is probably my error and not that of the plugin. Also, you have to learn a bit Jamal, but that may also be an advantage if you happen to like it. In short: you can give it a try if you fancy. Starting is easy since the tool (Jamal) is published in the central repo, the source and the documentation is on Github, thus all you need is to craft the genpom.xml file, cook some jam and start the plugin.

POM files are not the only source files that can be served with jam. I can easily imagine the use of Jamal macros in the product documentation. All you need is creating a documentationfile.md.jam file as a source file and modify the main POM to run Jamal during the build process converting the .md.jam to the resulting macro processed markdown document. You can also set up a separate POM just like we did in this article in case you want to keep the execution of the conversion strictly manual. You may even have java.jam files in case you want to have a preprocessor for your Java files, but I beg you not to do that. I do not want to burn in eternal flames in hell for giving you Jamal. It is not for that purpose.

There are many other possible uses of Jamal. It is a powerful macro language that is easy to embed into applications and also easy to extend with macros written in Java. Java::Geci also has a 1.0 version module that supports Jamal to ease code generation still lacking some built-in macros that are planned to make it possible to reach out to the Java code structure via reflections. I am also thinking about to develop some simple macros to read Java source files and to include into documentation. When I have some result in those I will write about.

If you have any idea what else this technology could be used for, do not hesitate to contact me.

What is var in Java

The var predefined type introduced in Java 10 lets you declared local variables without specifying the type of the variable when you assign a value to the variable. When you assign a value to a variable the type of the expression already defines the type of the variable, thus there is no reason to type the type on the left side of the line again. It is especially good when you have some complex long types with a lot of generics, for example

HashMap<String,TreeMap<Integer,String> myMap = mapGenerator();

Generic types you could already inherit in prior Java versions but now you can simply type

var myMap = mapGenerator();

This is simpler, and most of the times more readable than the previous version. The aim of the var is mainly readability. It is important to understand that the variables declared this way will have a type and the introduction of this new predefined type (not a keyword) does not render Java to be a dynamic language. There are a few things that you can do this way that you could not before or you could do it only in a much more verbose way. For example, when you assign an instance of an anonymous class to a variable you can invoke the declared methods in the class through the var declared variables. For example:

var m = new Object{ void z(){} }
m.z();

you can invoke the method z() but the code

Object m = new Object{ void z(){} }
m.z();

does not compile. You can do that because anonymous classes actually have a name at their birth, they just lose it when the instance gets assigned to a variable declared to be the type of Object.

There is a little shady part of the var keyword. This way we violate the general rule to instantiate the concrete class but declare the variable to be the interface. This is a general abstraction rule that we usually follow in Java most of the times. When I create a method that returns a HashMap I usually declare the return value to be a Map. That is because HashMap is the implementation of the return value and as such is none of the business of the caller. What I declare in the return type is that I return something that implements the Map interface. The way I do it is my own duty. Similarly, we declare usually the fields in the classes to be of some interface type if possible. The same rule should also be followed by local variables. A few times it helped me a lot when I declared a local variable to be Set but the actual value was TreeSet and then typing the code I faced some error. Then I realized that I was using some of the features that are not Set but SortedSet. It helped me to realize that sorted-ness is important in the special case and that it will also be important for the caller and thus I had to change the return type of the method also to be SortedSet. Note that SortedSet in this example is still an interface and not the implementation class.

With the use of var we lose that and we gain a somewhat simpler source code. It is a trade-off as always. In case of the local variables the use of the variable is close in terms of source code lines to the declaration, therefore the developer can see in a glimpse what is what and what is happening, therefore the “bad” side of this tradeoff is acceptable. The same tradeoff in case of method return values or fields is not acceptable. The use of these class members can be in different classes, different modules. It is not only difficult but it may also be impossible to see all the uses of these values, therefore here we remain in the good old way: declare the type.

The future of var (just ideas)

There are cases when you cannot use var even for local variables. Many times we have the following coding pattern:

Here we can not use var because there is no expression assigned to the variable on the declaration itself. The compiler, however, could be extended. From now on what I talk about is not Java as it is now. It is what I imagine how it can be in some future version.

If the structure is simple and the “do something here” is empty, then the structure can be transformed into a ternary operator:

In this case, we can use the var declaration even if we use an old version of Java, e.g.: Java 11. However, be careful!

var h = true ? 1L : 3.3;

What will be the actual type of the variable h in this example? Number? The ternary operator has complex and special type coercion rules, which usually do not cause any issue because the two expressions are close to each other. If we let the structure described above use a similar type coercion then the expressions are not that close to each other. As for now, the distance is far enough for Java not to allow the use of the var type definition. My personal opinion is that the var declaration should be extended sometime in the future to allow the above structure but only in the case when the two (or more in case of more complex structure) expressions have exactly the same type. Otherwise, we may end up having an expression that results in an int, another that results in a String and then what will the type of the variable be? Do not peek at the picture before answering!

(This great example was given by Nicolai Parlog.)

I can also imagine that in the future we will have something that is similar to Scala val, which is final var in Java 11. I do not like the var vs. val naming though. It is extremely sexy and geekish, but very easy to mistake one for the other. However, if we have a local variable declaration that starts with the final keyword then why do we need the var keyword after that?

Finally, I truly believe that var is a great tool in Java 11, but I also expect that it’s role will be extended in the future.